The present disclosure relates to methods of forming bi-metallic components for structural applications and, more particularly, to methodologies and technologies to achieving sound metallurgical bonding when liquid aluminum is cast over solid aluminum objects.
This section provides background information related to the present disclosure which is not necessarily prior art.
As vehicle weight reduction continues to be a driver in part design and development, various new strategies are being developed to provide strength at reduced weight. One strategy is the process of casting a light metal such as aluminum or magnesium onto a heavier metal substrate. Overcasting steel or copper with aluminum or magnesium allows one to take advantage of the strength of steel and the corrosion resistance and heat transfer capability of copper without compromising the light weight sought in many applications. Following the substitution of aluminum for ferrous castings in the automotive industry, further innovations involve adopting hybrid solutions where a mix of widely different materials are combined.
For instance, the high mechanical resistance of steel may be allied to the lightness of magnesium to create a hybrid assembly. One such example of a hybrid assembly used in automotive engines achieves weight reduction by casting magnesium over aluminum which, unlike magnesium, resists the corrosive aggression of the cooling fluid. Overcasting can be advantageous in reducing machining cost or enhancing heat transfer, such as by embedding copper pipes in aluminum. Similarly, inserts may be used in aluminum castings to locally enhance their strength, heat transfer properties or wear resistance. Aluminum and magnesium castings offer significant mass savings when compared with ferrous or copper parts. Hollow sections generally are more efficient in reducing mass in a mechanical assembly. These sections may be obtained by overcasting tubes of “heavy” materials with aluminum, which can accommodate the complexity in shape offered by the metal casting process and also meet the strength requirement.
Another example is the overcasting of the preformed conductor bars with aluminum to form the end rings in aluminum induction rotors. Casting single piece aluminum rotor (bars and end rings are all formed by liquid aluminum cast together) poses lot of challenges not only in casting process but also in the aluminum alloys used to make the rotors. Aluminum alloys used to cast rotor squirrel cages are usually high purity aluminum, or electric grade wrought alloys which are all difficult to cast because of their low fluidity, high shrinkage rate (density change from liquid to solid), high melting temperature and short solidification range, etc. These characteristics of the higher purity aluminum alloys increase porosity and the tendency of hot tearing, particularly at the locations where the conductor bars connect to the end rings, which leads to fracture between the conductor bars and the end rings. Furthermore, many cast aluminum squirrel rotor cages are made by high pressure die casting process in order to fill the thin and long bars (squirrel slots) in the laminate steel stack quickly to avoid cold shuts. The entrained air and abundant aluminum oxides produced during the high pressure die casting process, which are due to very high flow velocity (about 60 m/s) in mold filling, can not only decrease rotor quality and durability, but also significantly reduce the thermal and electric conductivity of the rotor, particularly in the conductor bars.
Bi-metallic casting techniques can be used to provide components having increased stiffness, strength, wear resistance, and other functionality. Bi-metallic casting allows two different metals to be combined in one component, while maintaining the distinct advantages offered by the constituent metals and/or alloys. In various bi-metallic casting techniques, at least a portion of base material or preform of a first metal or alloy is overcast with a second metal or alloy. Metal preforms may have an oxide layer or oxide film on their exterior substrate surface. Oxide layers may start as simple amorphous (non-crystalline) layers, such as Al2O3 on aluminum, MgO on magnesium and Mg—Al alloys, and Cu2O on copper. In certain aspects, their structures may derive from the amorphous melt on which they nucleate and/or grow and transform into complex and different phases and structures. The oxide layers may interfere with and/or negatively affect the ability of the metal preform to metallurgically bond with another metal under bonding conditions. Further, even if an oxide layer is once removed, there remains the possibility for another oxide layer to re-form under the appropriate oxidizing conditions and parameters. Thus, there remains a need for improved methods of forming even stronger metallurgical bonds between two metals joined using bi-metallic casting techniques.